Electrostatic lenses are used in applications such as ion implantation to control beam energy, focusing, and direction, among other operations. FIG. 1 shows a conventional ion implantation apparatus (ion implanter) 100 which comprises an ion source 102, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a magnet analyzer 110, and a an electrostatic lens 112. The D1 deceleration stage (also known as “deceleration lens”) is comprised of multiple electrodes with a defined aperture to allow an ion beam 114 to pass therethrough. By applying different combinations of voltage potentials to the multiple electrodes, the D1 deceleration lens can manipulate ion energies and cause the ion beam to hit a target wafer 116 at a desired energy. The electrostatic lens 112 may have multiple electrodes in the form of rods to which different combinations of voltage potentials may be applied to manipulate ion beam 114 as it passes therethrough. The above-mentioned deceleration stage 108 and the electrostatic lens 112 are typically electrostatic deceleration lenses.
FIG. 2a depicts a side view of a conventional electrostatic lens 200 using rod-shaped electrodes. in accordance with an embodiment of the present disclosure. The lens configuration of the electrostatic lens 200 using rod-shaped electrodes may also include several sets of electrodes, such as a set of entrance electrodes 202, one or more sets of suppression/focusing electrodes 204, and a set of exit electrodes 206. Each set of electrodes may have a space/gap to allow ions to pass therethrough with a central ray trajectory (“c.r.t.” or “crt”) of an ion beam 210 and at a deflection angle 95. The rod-shaped electrodes may be made of non-contaminating material, such as graphite, glassy carbon, and/or other non-contaminating material. It should be appreciated that the electrodes may also be made of materials with low thermal expansion coefficients
As illustrated in FIG. 2a the electrodes in the lens 200 using rod-shaped electrodes may be “flared” such that an opening for the ion beam 210 at the exit electrode 206 may be greater than an opening for the ion beam 210 at the entrance electrode 202. Accordingly, openings at each set of the suppression/focusing electrodes 204 may gradually increase or “flare” open. Also, symmetry may be maintained about the crt of the ion beam 210. The rod-shaped electrode rods may effectively provide independent control of deflection, deceleration, and/or focus during an ion implantation process. As further shown in FIG. 2a, a pump 212 may be directly or indirectly connected to the housing 214. In one embodiment, the pump 212 may be a vacuum pump for providing a high-vacuum environment or other controlled environment. In other embodiments, the housing 214 may include one or more bushings 216. These bushings 216 may be used to electrically isolate the housing 214 from other components.
In operation, the entrance electrode 202, the suppression/focusing electrodes 204, and the exit electrode 206 are independently biased such that the energy and/or shape of the ion beam 20 is manipulated in the following fashion. The ion beam 210 may enter the electrostatic lens 200 through the entrance electrode 202 and may have an initial energy of, for example, 10-20 keV. Ions in the ion beam 210 may be accelerated between the entrance electrode 202 and the suppression/focusing electrodes 204. Upon reaching the suppression electrodes 204, the ion beam 210 may have an energy of, for example, approximately 30 keV or higher. As the ion beam 210 propagates between the suppression/focusing electrodes 204 and the exit electrode 206, the ions in the ion beam 210 may be decelerated, typically to an energy that is closer to the one used for ion implantation of a target wafer. In one example, the ion beam 210 may have an energy of approximately 3-5 keV or lower when it exits the electrostatic lens 200.
In certain applications, particularly those requiring implantation of large workpieces, such as 300 mm wafers or larger as the implantation target, it is advantageous to generate ion beams in the form of ribbon-shaped beams having high aspect ratios such that the cross-section of the beam is much larger in one dimension (W) than the other (H) as is illustrated in FIG. 2b. These ribbon beams are commonly used in ion implanter apparatus and implantation systems where a single workpiece such as a silicon wafer or flat panel display is moved in a single dimension through the ion beam. In these instances, the cross-section of the width W of the ribbon ion beam, that is longer dimension of the ion beam cross-section, is much larger than the length H.
In order to produce a stable ion beam, it is important that beamline components such as an electrostatic lens operate in a stable fashion. Conventional electrostatic lens components are constructed from a material such as graphite, which provides low contamination to a beam passing through the electrostatic lens. However, upon occasion, the graphite rods may vibrate at a natural frequency that is characteristic of the length of the material. The natural frequency refers to the frequency at which an element, such as a rod or beam, vibrates once the element has been set into motion.
Referring again to FIG. 2a, in the case where electrodes 204 are graphite rods, this vibration causes a fluctuation dS in the separation S of the electrodes 204 in a direction (Y) perpendicular to the direction of propagation (Z) of the ion beam 210 passing between the sets of electrodes 202-206. The fluctuation in separation dS in turn leads to fluctuations in the electric field strength experienced by the ion beam 210 passing through the electrostatic lens. In particular, the electric field fluctuation in the electrostatic lens may take place according to the vibration frequency of the electrostatic lens electrodes. Referring also to FIG. 2b, such fluctuations in the electric field may perturb the ion beam 210 in a manner that causes spatial fluctuations in the ion beam 210 in the Y-direction leading to a less uniform ion beam.
During ion implantation, a wafer may be moved with respect to the ion beam and any non-uniformity in the ion beam caused by such fluctuations may be averaged out, leading to a uniformly implanted wafer. However, occasionally, the non-uniformity of the ion beam caused by vibrations in the electrostatic lens may express itself as micro-striping on the wafer, which denotes the generation of non-uniform ion dose on the wafer, where each stripe represent a region of ion dose that is different from an adjacent region. Referring to FIGS. 2b and 2c, if the wafer is moved along path 220 along the Y-direction with respect to the ion beam 210, the micro-striping may form stripes 222 where the ion dose varies between different stripes. As shown in FIG. 2c, the long axis of the stripes may be parallel to the long axis of the ion beam 210, that is, parallel to the X-direction. This effect may be observable in particular when the vibration frequency of the electrodes is relatively low and the rate of wafer movement is relatively high. In principle, the micro-striping effect may be reduced or eliminated by moving the wafer with respect to the ion beam at a rate sufficiently low as to permit the ion beam fluctuations to cancel one another out within each region of the substrate that is implanted. However, the reduced rate of movement of the wafer needed to eliminate the microstriping may lead to a reduced throughput rate for substrates. Moreover, as the dimension of substrates increase, it may be useful to provide wider ribbon beams, and wider electrostatic lenses to process such ion beams, leading to rods that may extend beyond one meter in length. At this dimension, the micro-striping problem caused by rod vibration may be exacerbated due to lower vibration frequency and/or greater fluctuation dS in the rods. In view of the above, it will be apparent that improvements in electrostatic lens construction are needed.